The present invention relates generally to computing circuits, and more particularly, to a computing circuit for producing a digital output representing the ratio between a selected pair of analog signals. Circuits capable of producing a digital output representing the ratio between a pair of analog signals are known. One such circuit is disclosed in U.S. Pat. No. 3,752,972, issued to Campbell, Jr., on Aug. 14, 1973, and requires the simultaneous supply of a reference analog signal and a test analog signal to the computing circuit in order to produce an output representing the ratio between the two signals.
The reference analog signal must remain constant for each test to provide ratio readings that are consistent. For example, if it is desired to determine whether a given characteristic of a workpiece is within a selected percentage of the given characteristic of a selected reference workpiece, the reference analog signal representing that characteristic must be continually generated and supplied to the computing circuit along with the analog signal of the tested workpiece. If during the testing of successive workpieces the tested characteristic should be altered on the selected reference workpiece due to external conditions such as heat, etc., an unreliable ratio will be produced which may result in the rejection of good workpieces and the acceptance of workpieces which do not conform to the desired standard. Thus, computing circuits requiring more than one test input have inherent disadvantages.
It is therefore the object of the present invention to provide a computing circuit having a memory function for retaining a value representing a selected analog signal.